Fiber Reinforced Cellular Foam Product

ABSTRACT

Fiber reinforced polymer foam articles are disclosed. Through the system and process of the present disclosure, fibers can be incorporated into the foam article during an injection molding process while minimizing fiber breakage. Thus, foam articles can be produced having relatively long fibers. For instance, when using a feed stock containing fibers having a length greater than about 0.7 cm, such as from about 1 cm to about 1.3 cm, foam articles can be produced in which at least about 10% by volume of the fibers have a length greater than 3 mm and wherein at least about 1% by volume of the fibers have a length greater than about 7 mm.

BACKGROUND

Fiber reinforced thermoplastic composites generally include reinforcingfibers contained in a polymer matrix made from a thermoplastic polymer.The presence of the fibers can greatly increase certain mechanicalproperties of the polymer. The thermoplastic polymer, on the other hand,is capable of being formed into any suitable shape. Thus, many shapedparts in different industries can be made from polymer compositematerials.

In one embodiment, shaped or molded parts made from polymer compositescan be prepared from pellets of coated, long fiber reinforced compositestructures. U.S. Pat. No. 6,090,319, for instance, which is incorporatedherein by reference, describes generally a production process whereincontinuous fibers are coated in a die and then cut to the desired lengthto form the pellets. The pellets are made according to a pultrusionprocess and can include any suitable thermoplastic polymer, such as apolyolefin, a polyamide, or mixtures thereof. Composite pellets are alsodisclosed in U.S. Pat. No. 6,844,059, U.S. Pat. No. 6,794,032, and U.S.Pat. No. 6,482,515, which are all incorporated herein by reference.

Shaped parts or structures can be formed from the composite pelletsusing any suitable process. In one embodiment, for instance, the shapedparts can be made through an injection molding process. In an injectionmolding process, the polymer and fiber material is heated above thesoftening temperature of the polymer and the resulting fluid polymermaterial is introduced into a mold in a manner that causes the polymermaterial to assume the interior shape of the mold.

In one particular embodiment, the composite polymer material is formedinto a microcellular plastic foam during the injection molding process.For example, a blowing agent may be mechanically or chemicallyintroduced into the polymer melt during the process which causes thefoam to form. The blowing agent may comprise, for instance, asupercritical fluid. For example, U.S. Pat. No. 6,884,377 and U.S.Patent Application Publication No. 2005/0042434, which are incorporatedherein by reference, disclose injection molding processes for producingpolymer foam articles.

As stated in the '434 application, although the introduction ofreinforcing fibers into molded polymer foam articles is known, oneproblem that has been encountered in the injection molding of thesearticles is that the fibers can break during the process, which cancompromise the properties of the resulting articles. Thus, the '434application is directed to injection molding polymer composite articlesthat results in less breakage of the reinforcing fibers. Although theprocess disclosed in the '434 application has shown to create lessbreakage of the reinforcing fibers, the present disclosure is directedto further improvements in injection molding processes for producingfiber reinforced foam polymer articles.

SUMMARY

The present disclosure is generally directed to fiber reinforced polymerfoam articles. As will be described in greater detail below, through theprocess of the present disclosure, fiber reinforced polymer foamarticles can be produced in which the articles contain great amounts ofrelatively long fibers. By maintaining relatively long fiber lengths,reinforced polymer foam articles can be produced having enhancedmechanical properties.

For example, through the process of the present disclosure, a polymercomposite article can be produced that includes a polymer matrix madefrom a thermoplastic polymer having a cellular structure. Fibers can bedispersed in the polymer matrix. The fibers, in one embodiment, can bepresent in the polymer matrix in an amount of at least about 10% byweight and can have a length characterized in that at least 10% of thefibers by volume have a length greater than about 3 mm, such as fromabout 3 mm to about 4 mm.

In one embodiment, at least some of the fibers contained in the polymermatrix can have a length greater than about 6 mm. For instance, at least1% of the fibers by volume can have a length greater than about 7 mm.The above lengths can be achieved when forming the polymer compositearticle from pellets containing fibers having a length of greater thanabout 0.7 cm.

The average fiber length of the fibers contained in the polymercomposite article can vary depending upon various factors, including thestarting material and the process conditions. The average fiber length,for instance, can be greater than about 1.25 mm, such as greater thanabout 1.3 mm, such as greater than about 1.4 mm, such as even greaterthan 1.5 mm.

The thermoplastic polymer used to form the polymer composite article cancomprise any suitable polymer or polymer blend. In one embodiment, forinstance, the thermoplastic polymer may comprise a polyolefin, apolyamide, or mixtures thereof. As used herein, the term “polyolefin”,the term “polyamide” or any other similar polymer class includeshomopolymers, copolymers, terpolymers, and the like. Polyolefins thatmay be used in the process according to the present disclosure includepolyethylene, polypropylene, and mixtures thereof.

In general, any suitable fiber can be combined with the thermoplasticpolymer to form the composite article. In one embodiment, for instance,the fibers may comprise glass fibers. Other fibers that may be usedinclude talc fibers, wollastonite fibers, carbon fibers, metal fibers,aromatic polyamide fibers, and mixtures thereof.

As described above, the polymer composite article has a cellularstructure. For instance, the structure can include open cells and/orclosed cells. In one embodiment, for instance, the article can have avoid volume of at least about 5%, such as at least about 10%. The cellscan have any suitable size, such as having an average cell size of lessthan about 100 microns. The cell density can be at least about 10⁶ cellsper cubic centimeter.

The polymer composite articles made in accordance with the presentdisclosure can be made using an injection molding process. A speciallydesigned screw can be contained in the injection molding equipment thatassists in preserving the fiber length. Ultimately, polymer compositearticles can be formed that have a drop impact of greater than about 7ft-lbs, such as greater than about 8 ft-lbs. The polymer compositearticle can also have a notched impact of greater than about 2.5ft-lbs/in². In addition, the polymer composite article can have aflexural strength of greater than about 19,000 psi, such as greater thanabout 19,500 psi and can a tensile strength of greater than about 12,000psi, such as greater than about 12,500 psi.

Other features and aspects of the present disclosure are discussed ingreater detail below.

BRIEF DESCRIPTION OF THE DRAWINGS

A full and enabling disclosure of the present invention, including thebest mode thereof to one skilled in the art, is set forth moreparticularly in the remainder of the specification, including referenceto the accompanying figures, in which:

FIG. 1 is a cross sectional view of an injection molding system that maybe used to produce fiber reinforced polymer foam articles in accordancewith the present disclosure; and

FIG. 2 is a graphical representation of some of the results obtained inthe Example described below.

Repeat use of reference characters in the present specification anddrawings is intended to represent the same or analogous features orelements of the present invention.

DETAILED DESCRIPTION

It is to be understood by one of ordinary skill in the art that thepresent discussion is a description of exemplary embodiments only, andis not intended as limiting the broader aspects of the presentinvention.

In general, the present disclosure is directed to fiber reinforcedpolymer foam articles having improved properties. The foam articles canbe produced according to an injection molding process and can bereinforced using any suitable fiber, such as glass fibers. Through theprocess of the present disclosure, fibers can be incorporated into thepolymer foam articles in a manner that minimizes fiber breakage duringthe injection molding process. Consequently, polymer foam articles canbe produced having relatively long fiber lengths which serve to enhancethe mechanical properties of the resulting articles. In one embodiment,the fiber reinforced polymer foams can be made by mechanically orchemically dispersing a blowing agent into a composite polymer meltduring the injection molding process. The blowing agent, for instance,can be a gas such as carbon dioxide or nitrogen. In one embodiment, theblowing agent may comprise a supercritical fluid.

A supercritical fluid is a gas that exists above its critical point. Thecritical point of a gas is the highest temperature and pressure at whicha substance can exist as a vapor and liquid in equilibrium. Carbondioxide at a pressure of about 1100 psi or nitrogen at a pressure ofabout 750 psi becomes supercritical and dissolves into the polymer melt.As the molding pressure decreases, the gas undissolves to form a foamhaving a cellular structure. The cellular structure can be open orclosed. The cell sizes can vary but are generally less than about 100microns, such as from about 5 microns to about 100 microns.

Fiber reinforced polymer foams made in accordance with the presentdisclosure have numerous applications and uses. For instance, since thefiber reinforced foam composites can be formed into any suitable shape,the composites can be used to form various different products andvarious different parts to be used in numerous systems. For instance, inone embodiment, the foam composites of the present disclosure can beused to form automobile parts, sound insulation materials, or may beused in aircraft applications, in marine applications, etc.

The fiber reinforced polymer foam articles of the present disclosure areformed through an injection molding process using a specially designedscrew that is intended to minimize fiber breakage. As will be describedin greater detail below, fiber breakage is also minimized by carefullycontrolling certain process conditions.

Referring to FIG. 1, for instance, one exemplary embodiment of aninjection molding system generally 10 that may be used to form fiberreinforced polymer foam articles in accordance with the presentdisclosure is illustrated. As shown, the molding system 10 includes ascrew 12 contained within a barrel 14. The barrel 14 includes a firstend 16 and a second end 18. The barrel 14 is in communication with ahopper 20 towards the first end 16 and in communication with a moldingcavity 22 towards the second end 18. The screw 12 is in operativeassociation with a drive motor 24 that causes the screw to rotate.

During the molding process, polymer pellets containing reinforcingfibers are placed into the hopper 20 and are introduced into the barrel14 through an opening 26. Within the barrel 14, the polymer pellets areheated into a molten state. The drive motor 24 rotates the screw 12which then, in turn, pushes the molten polymer composite material downthrough the barrel and into the molding cavity 22.

In order to heat the composite polymer within the barrel 14, the barrel14 can be in communication with any suitable heating device. Forinstance, the barrel 14 can be heated through electrical resistanceheaters, gas heaters, and the like. In one embodiment, the heatingdevice that heats the barrel 14 can be controlled so that differentzones of the barrel are at different temperatures. In this regard, thebarrel 14 can be in communication with a plurality of temperaturecontrol units 28. The temperature control units, for instance, canmonitor the temperature of the barrel 14 and can send information to acontroller, such as a microprocessor or programmable logic unit. Thecontroller, in turn, can control the heating device for maintaining thetemperature of the barrel at the various locations within presettemperature limits. The temperature control units can work inconjunction with a controller in a closed loop manner or in an open loopmanner.

The composite polymer pellets contained in the hopper 20 and fed to thebarrel 14 can include any suitable polymer material. The polymermaterial, for instance, may comprise a single polymer or may comprise acombination or blend of polymers. In general, the polymer containedwithin the composite pellets comprises a thermoplastic polymer. Thethermoplastic polymer may be a homopolymer, a copolymer, a terpolymer,and the like. The thermoplastic polymer may also be amorphous,semicrystalline, or crystalline.

Suitable thermoplastic polymers that may be used to construct thecomposite polymer pellets include, for instance, any suitablepolyolefin. Examples of polyolefins include homopolymers and/orcopolymers of high, medium, or low density polymers, such aspolyethylene, polypropylene, polymethylpentene, and copolymers of theabove. The homopolymers and copolymers may be straight-chain orbranched. In one embodiment, for instance, a semicrystalline homopolymerof an alpha-olefin and/or ethylene, or copolymers of these may be used.In one particular embodiment, polypropylene is contained within thecomposite polymer pellets.

In other embodiments, the thermoplastic polymer may comprise apolyamide, such as any suitable nylon. Other thermoplastic polymersinclude polyesters, polyimides, fluoropolymers, polyvinyl chloride,polyaromatics, and styrenic polymers. Styrenic polymers includepolystyrene, ABS rubbers, block copolymers, and the like. The polymercontained within the composite pellets may also comprise ametallocene-catalyzed polyolefin, such as a polyethylene, that may beconsidered a thermoplastic elastomer.

In general, thermoplastic polymers that may be used to form the fiberreinforced polymer articles can have a melt flow rate of less than about40, such as those having a melt flow rate of less than about 10.

The fiber contained within the composite polymer pellets can also varydepending upon the particular application and the desired result. Ingeneral, any suitable reinforcing fiber may be used. For example, in oneembodiment, glass fibers may be used as the reinforcing fibers. Otherreinforcing fibers that may be used in accordance with the presentdisclosure include talc fibers, wollastonite fibers, carbon fibers,metal fibers, aromatic polyamide fibers (e.g. KEVLAR), and fibers madefrom aromatic liquid crystalline polymers (e.g. VECTRA).

Fiber diameters and fiber lengths of the fibers contained within thecomposite pellets can also vary. In general, for instance, thereinforcing fibers can have a diameter of less than about 500 microns,such as less than about 250 microns, such as less than about 100microns. For instance, when using glass fibers, the fibers can have afiber diameter of from about 8 microns to about 25 microns and can havea weight of from about 500 to about 4400 grams per 1000 m. If desired,the fibers can also be pretreated with a sizing that may facilitatemixing with the polymer material.

The length of the fibers contained within the composite polymer pelletscan, in one embodiment, be relatively long or, in other embodiments, maybe relatively short. The present disclosure, however, is particularlywell suited to minimizing fiber breakage when using relatively longfibers. In this regard, the initial fiber length can be greater thanabout 0.7 cm, such as greater than about 1 cm. For instance, the fiberscan have a length of from about 0.7 cm to about 2 cm, such as from about1 cm to about 1.5 cm. In other embodiments, however, relatively shortfibers may be contained within the composite polymer pellets. Forinstance, the fibers can have a length of less than about 0.7 cm, suchas from about 0.1 cm to about 0.5 cm.

The amount of fibers contained within the composite polymer can alsovary depending upon various factors. For instance, the amount of fiberscontained within the polymer composite can depend upon the type offibers used, and the end use application for the resulting foam article.In general, the fibers can be contained within the composite polymer inan amount from about 5% to about 80% by weight, such as from about 10%to about 70% by weight. In one particular embodiment, for instance, thefibers can be contained within the composite polymer in an amount fromabout 30% by weight to about 50% by weight.

Composite polymer pellets that may be used in accordance with thepresent disclosure can be obtained from various commercial sources. Forexample, the Cellanese Corporation of Dallas, Tex., markets variousdifferent fiber reinforced polymer pellets under the name CELSTRAN.CELSTRAN PP-GF40-02-04, for instance, comprises composite polymerpellets containing 40% by weight glass fibers contained in apolypropylene matrix. CELSTRAN PA6-GF50-03, on the other hand, contains50% by weight glass fibers contained in a polyamide matrix.

In one embodiment, the composite pellets are formed by impregnatingfiber rovings and then cutting the impregnated fiber rovings to adesired size. In this manner, chopped fibers become incorporated intothe polymer pellets. The resulting pellets, for instance, have arod-like shape having a length of from about 0.7 cm to about 2 cm andhaving a diameter of from about 1 mm to about 10 mm.

In the embodiment illustrated in FIG. 1, as described above, the hopper20 is intended to contain the composite polymer pellets. It should beunderstood, however, that in other embodiments a molten polymer materialcan be added directly into the barrel 14.

In addition to one or more polymers and the reinforcing fibers, thepolymer composite fed through the barrel 14 can also contain variousother additives. Other additives may include, for instance, lubricants,dyes, pigments, antioxidants, heat stabilizers, light stabilizers,particulate reinforcing agents, fillers, hydrolysis stabilizers, and thelike.

In order to form a cellular or foam product, the molten polymercomposite material moved through the barrel 14 is combined with ablowing agent prior to being fed to the molding cavity 22. In thisregard, the barrel 14 can be placed in communication with a blowingagent delivery system generally 30. As shown, the blowing agent deliverysystem 30 includes a blowing agent supply 32 in communication with apressure and metering device 34. From the blowing agent supply 32, ablowing agent is fed into the barrel 14 through at least one port 36. Asshown, the barrel 14 can include a plurality of ports 36. For example,in the embodiment illustrated, the blowing agent delivery system 30includes three ports 36. Each of the injection ports 36 may, if desired,be in communication with a shutoff valve which allow the flow of theblowing agent into the extruder barrel 14 to be controlled as a functionof axial position of the rotating screw 12.

In general, any suitable blowing agent may be used in the process. Theblowing agent, for instance, may comprise a physical blowing agent or achemical blowing agent. Examples of suitable blowing agents include, forinstance, hydrocarbons, chlorofluorocarbons, nitrogen, carbon dioxide,helium, and the like.

In one embodiment, the blowing agent may comprise a supercritical fluid.Supercritical fluids that may be used include, for instance, carbondioxide, nitrogen, or combinations thereof. Supercritical fluids can beintroduced into the barrel and made to rapidly form a single-phasesolution with the polymer composite material either by injecting theadditive as a supercritical fluid, or injecting it as a gas or liquidand allowing conditions within the extruder to render it supercritical.

Combining a supercritical fluid with the composite polymer materialproduces a single-phase solution having a very low viscosity whichadvantageously allows lower temperature molding, as well as rapidfilling of molds having close tolerances to form very thin molded parts,parts with very high length to thickness ratios, parts including thickerdistal regions, molding carried out at low clamp force, and the like.

The supercritical fluid thus not only reduces the viscosity of themolten polymer material but also serves as a blowing agent. Using asupercritical fluid also allows for the control of the resultingproperties of the foam. In particular, cellular, and particularlymicrocellular, articles can be produced having a void volume and/or acell size and/or a cell density within controlled limits. All of theseadvantages can be obtained while using a relatively low amount of thesupercritical fluid. For instance, the supercritical fluid can bepresent in the composite polymer material in an amount less than about10% by weight, such as less than 5% by weight, such as less than 1% byweight, such as even less than about 0.5% by weight.

As mentioned above, the supercritical fluid allows for the injection ofthe composite polymer material into the mold cavity 22 at reducedtemperatures. For instance, injection can take place at a moldingchamber temperature of less than about 100° C., such as less than about75° C., such as less than about 50° C., such as less than about 30° C.,or even less than about 10° C.

The pressure and metering device 34 is positioned in between the blowingagent supply 32 and the at least one port 36. The pressure and meteringdevice 34 can be used to meter the mass of the blowing agent, such asbetween about 0.01 lbs/hr to about 70 lbs/hr.

The particular blowing agent used and the amount of blowing agentincorporated into the composite polymer material can be selected so asto produce a foamed product with the desired cell size and void volume.

As shown in FIG. 1, the one or more ports 36 are located within orupstream from a mixing section 38 of the screw 12. The ports 36 can belocated at different locations along the barrel. In one embodiment, forinstance, two ports may be positioned on opposing top and bottom sidesof the barrel 14. A blowing agent entering the barrel 14 through theports 36 rapidly and evenly mixes with the molten composite polymermaterial into a fluid polymer stream. When the blowing agent is asupercritical fluid, a single-phase solution is produced. Having aplurality of ports that are positioned radially around the barrel 14 mayenhance mixing. Further, it should be understood that many more ports 36may be positioned along the barrel 14.

As shown in FIG. 1, the screw 12 contained within the barrel 14 includesa first portion of flights or threads that are unbroken and a secondportion 38 containing broken threads. In addition, the screw 12 caninclude a check valve 40 that separates a first section from a secondsection.

In one embodiment, the ports 36 are located opposite unbroken flightsalong the screw 12. In this manner, as the screw rotates, each flightpasses or wipes each port periodically. This wiping increases rapidmixing of the blowing agent with the composite molten polymer material.In particular, the flights rapidly open and close each port as the screw12 rotates. The result is the distribution of relatively finely-divided,isolated regions of blowing agent in the fluid polymer materialimmediately upon injection and prior to any mixing.

Once the blowing agent is combined with the composite molten polymermaterial, the resulting mixture is then fed through the mixing section38 contained within the barrel 14. In the mixing section, the blowingagent becomes intimately mixed with the polymer. As described above,when a supercritical fluid is present, the fluid dissolves within thepolymer.

As shown in FIG. 1, the mixing section 38 includes a plurality of brokenflights. More particularly, the flights include spaced apart gaps. Thegaps allow better mixing of the components.

In accordance with the present disclosure, the mixing section isparticularly well designed to prevent against fiber breakage. Inparticular, the screw 12 includes less than six flights between the endof the screw and the ports 36. For example, the screw 12 can includethree to five flights, such as four flights within the mixing section.In addition, the gaps contained within the flights within the mixingsection 38 have a greater length than similar screws designed in thepast. In particular, the gaps have a length of at least about 10 mm,such as from about 12 mm to about 20 mm.

Providing the relatively wide gaps in the flights and having the gapsspaced as described above has been found to prevent fiber breakage inconjunction with using a relatively low number of flights within themixing section.

In addition to the above, the screw 12 also contains various otherunique features that are designed to prevent fiber breakage. Forexample, the check valve 40, in one embodiment, can be a ring checkvalve. In the past, for instance, similar screws included ball checkvalves. A ring check valve as shown in FIG. 1, however, has been foundto provide improved process control.

In addition, the screw 12 has a relatively low compression ratio. Forexample, the compression ratio of the screw 12 is generally less thanabout 2.5:1, such as less than about 2.3:1, such as less than about2.1:1. For instance, in one embodiment, the screw 12 can have acompression ratio of about 2:1.

Through the design of the screw 12, intimate mixing with the blowingagent is achieved within the barrel 14 while minimizing shear forcesbeing subjected on the polymer material. Ultimately, the screw 12 iscapable of combining the composite molten polymer mixture with theblowing agent, such as a supercritical fluid, while minimizing fiberbreakage.

After the composite molten polymer material and the blowing agent arecombined together, as shown in FIG. 1, the resulting mixture enters anaccumulation region 42. In the accumulation region 42, the temperatureof the mixture can be carefully controlled along with other processconditions. When using a supercritical fluid as a blowing agent, asingle-phase, non-nucleated solution of polymer material and blowingagent containing fibers is accumulated prior to being injected into themolding cavity 22.

From the accumulation region 42, the mixture enters a nucleator 44constructed to include a pressure-drop nucleating pathway 46. Thepressure of the polymer fiber and blowing agent mixture drops below thesaturation pressure for the particular blowing agent concentration at arate or rates facilitating nucleation. Nucleation is a process by whicha homogeneous, single-phase solution of polymer material, in which isdissolved molecules of a species that is gas under ambient conditions,undergoes formations of clusters of molecules of the species that definenucleation sites from which cells grow to form a foam. Duringnucleation, a homogeneous, single-phase solution changes to a mixture inwhich sites of aggregation of at least several molecules of blowingagent are formed. Nucleation defines that transitory state when gas, insolution in a polymer melt, comes out of solution to form a suspensionof bubbles within the polymer melt. When using a supercritical fluid,this transition occurs by changing the solubility of the blowing agentwithin the polymer. Nucleation occurs in the process of the presentdisclosure through a rapid temperature and/or pressure drop.

The nucleator 44 as shown in FIG. 1 can be located at differentlocations within the injection molding system. In the embodiment shownin FIG. 1, for instance, the nucleator 44 defines a nozzle connectingthe barrel 14 to the molding cavity 22. Thus, the nucleator defines anopening 48 that releases the blowing agent, fiber and polymer mixtureinto the molding cavity 22.

The opening 48 and the pathway 46 can have any size sufficient for afoam to form within the molding cavity 22. In one embodiment, thepathway 46 and the opening 48 can be adjustable in order to achieve adesired nucleation density. Further, while the pathway 46 defines anucleating pathway, some nucleation may also take place within themolding cavity itself as pressure on the polymer material drops at avery high rate during filling of the mold.

Injection of the molten composite polymer material and blowing agentinto the molding cavity 22 results in the production of a cellularmaterial that may be classified as a foam. During injection of thematerial into the molding cavity 22, cell growth occurs. If desired, themolding cavity 22 can include vents to allow gas escape duringinjection.

In the embodiment illustrated in FIG. 1, the accumulation region 42 isshown located within the barrel 14. In an alternative embodiment,however, a separate accumulator may be provided. In this embodiment, thepolymer material, fibers and blowing agent can be fed to a separateaccumulator prior to being injected into the molding cavity 22.

As described above, the injection molding system 10 includes a uniquelydesigned screw 12 that is intended to minimize fiber breakage. Variousother process parameters may also be carefully controlled in order tominimize any fiber damage. For instance, during the process, in oneembodiment, the screw may rotate at a relatively low speed. Forinstance, the screw may rotate at a speed of less than about 70 rpm,such as less than about 60 rpm, such as from about 40 rpm to about 60rpm. For instance, in one embodiment, the screw may rotate at a speed ofabout 50 rpm.

In addition, the present inventors have found that, in one embodiment,uniform results can be achieved when maintaining a relatively low backpressure within the barrel 14. The back pressure is the pressure (suchas hydraulic pressure) applied to the back of the molding screw toresist the force of the screw pushing back as the plasticized compositeresin is moved forward as the screw rotates during plasticization. Inthe process of the present disclosure, the pressure forcing the screwback is due primarily due to the pressurized blowing agent, such assuper critical fluid in the forward chamber of the screw. This pressurerequires a counter pressure to prevent the screw from being pushedbackwards, which would release pressure on the gas and result inpremature foaming of the polymer. The back pressure, for instance, canbe less than about 2000 psi, such as less than about 1500 psi, such asless than about 1250 psi. For instance, in one embodiment, the backpressure can be maintained between about 800 psi and about 1200 psi,such at around about 1000 psi.

Ultimately, through the use of the screw 12 and the process conditions,cellular fiber reinforced polymer articles can be produced havingenhanced properties. The foam articles, for instance, can have an opencellular structure or a closed cellular structure. In general, the voidvolume can be from about 1% to about 50%, such as from about 3% to about25%. For instance, in one embodiment, the void volume can be from about5% to about 15%. The average cell size can vary depending upon differentprocess conditions. In general, the cell size is less than about 100microns. The cell density, on the other hand, can be at least about 10⁶cells per cubic centimeter.

The size of the fibers contained in the resulting polymer matrix canalso vary depending upon process conditions, the size of the fiberscontained in the initial mix, and various other factors. When the fibershave an initial length of greater than about 0.7 cm, for instance, afiber reinforced polymer foam article can be produced wherein at least10% of the fibers by volume have a length greater than about 3 mm, suchas from about 3 mm to about 4 mm. Of particular advantage, fiberspresent in the matrix can have a length greater than about 6 mm, such asgreater than about 6.5 mm, such as greater than about 7 mm, such as evengreater than about 7.5 mm. For instance, when the initial fiber lengthis greater than about 0.7 cm, such as from about 1 cm to about 1.3 cm,about 1% of the fibers by volume in the resulting foam article can havea length greater than about 7 mm, such as greater than about 7.5 mm.

The average fiber length of foam articles made in accordance with thepresent disclosure can be greater than about 1 mm, when the initialfiber length is greater than about 0.7 cm, such as from about 1 cm toabout 1.3 cm. For instance, in one embodiment, the average fiber lengthcan be greater than about 1.1 mm, such as greater than about 1.2 mm,such as greater than about 1.25 mm. In other embodiments, even greateraverage fiber lengths can be achieved. For instance, the average fiberlength can be greater than about 1.3 mm, such as greater than about 1.35mm, such as greater than about 1.4 mm, such as greater than about 1.45mm, such as even greater than about 1.5 mm.

Due in large part to the greater fiber lengths, various mechanicalproperties of the resulting foam article are greatly enhanced. Forinstance, the foam article can have a drop impact of greater than about7 ft-lbs, such as greater than about 7.5 ft-lbs, such as even greaterthan about 8 ft-lbs. In addition, the foam article can have a notchedimpact of greater than about 2.4 ft-lbs/in², such as greater than about2.5 ft-lbs/in². The foam article can have a flexural strength of greaterthan about 19,000 psi, such as greater than about 19,500 psi, such asgreater than about 20,000 psi. The flexural strain of the article can beless than about 3.25%.

Fiber reinforced polymer foam articles made in accordance with thepresent disclosure can also have a tensile strength of greater thanabout 11,500 psi, such as greater than about 11,750 psi, such as greaterthan 12,000 psi. In fact, the tensile strength of foam articles made inaccordance with the present disclosure can be greater than about 12,250psi, such as greater than about 12,500 psi, such as even greater thanabout 12,600 psi. The tensile strain of the article, on the other hand,can be less than about 2.5%, such as less than about 2.4%, such as evenless than about 2.3%.

Polymer foam articles according to the present disclosure even performwell under conditions of high temperature and high strain. For instance,polymer foam articles made according to the present disclosure can havea heat deflection temperature of greater than about 149° C., such asgreater than about 149.5° C.

Of particular advantage, fiber reinforced, polymer foam articles made inaccordance with the present disclosure also have reduced warpage. Thereduced warpage is particularly significant when producing polymerarticles having relatively long lengths in relation to relatively smallthicknesses. Although unknown, it is believed that the design of thescrew in conjunction with the process conditions allow the resultingpolymer, fiber and blowing agent mixture to fill molding cavitieswithout stresses and non-uniform shrinkages often experienced in thepast.

The present disclosure may be better understood with reference to thefollowing example.

EXAMPLE

The following example demonstrates the improvements obtained whenproducing polymer composites in accordance with the present disclosurein comparison to past methods. In particular, in this example, compositefoam polymers were produced according to the present disclosure usingthe injection molding system as generally illustrated in FIG. 1. Inaddition, samples were also produced generally using the injectionmolding system as disclosed in U.S. Patent Application Publication No.2005/0042434.

Each set of samples was produced from glass fiber reinforcedpolypropylene pellets. A super critical fluid was used as a blowingagent in order to form a cellular composite product. Each set of sampleswere then subjected to various standard tests to determine theirproperties. In addition, fiber lengths in the resulting compositeproducts were studied.

The following tests were conducted on the samples:

Drop Impact Test:

The drop impact test determines the ability of the sample to absorb animpact and to measure toughness. The drop impact test was testedaccording to ASTM Test No. 3763. Results are measured in units energyand represent the amount of energy necessary to cause a failure of thesample.

Notched Impact Test:

According to the notched impact test, a notch is placed into the polymerspecimen. A striking member then strikes the specimen where the notchhas been made. The test was conducted according to ASTM Test No. D256.Results are reported in the energy necessary to cause failure of thespecimen.

Flexural Strength:

The flexural strength of a sample is defined as its ability to resistdeformation under load. More particularly, the flexural test isconducted according to ASTM Test No. D790 and measures the forcerequired to bend the specimen under 3 point loading conditions. Forinstance, the specimen is placed on a support span and a load is appliedto the center. The flexural strength of the sample is measured as theamount of pressure needed to cause 5 percent deformation. Flexuralstrain is also recorded as a percentage. The flexural strength andstrain tests were conducted according to ASTM Test No. D790.

Tensile Strength and Tensile Strain:

The tensile strength and tensile strain properties of the samples weretested according to ASTM Test No. D638.

Heat Deflection Temperature Test (HTD):

The heat deflection temperature is the temperature at which a standardspecimen deflects a specified distance under a load. Specimens, forinstance, are lowered into a bath where the temperature is raised at 2°C. per minute until the specimen deflects 0.25 mm. The heat deflectiontemperature test was conducted according to ASTM Test No. D648.

In addition to the above tests, a volume weighted fiber lengthdistribution was also generated for each sample. The process used todetermine fiber length distribution by volume is described and disclosedin U.S. Provisional Patent Application No. 60/825,200 filed on Sep. 11,2006, and herein incorporated by reference. According to the procedure,a sample of the fiber reinforced polymer foam article is removed fromthe article. During removal from the article, some of the fibers may becut. The sample preparation method used is an attempt to separate thefibers that have not been cut from those that have. The cut fibers arethen discarded. The remaining sample is evaluated for entangled fibers.If entangled fibers are present, they are gently untangled using wireprobes. The fibers are then randomized and placed in a Petri dish forimage analysis.

A specific procedure for sample preparation is as follows:

-   -   (1) Cut a 1″ inch square from the area of interest;    -   (2) Place sample in crucible and ash using a muffle furnace at        450° C. over-night (this temperature does not embrittle the        fibers);    -   (3) Use an anti-static device to ensure that the glass sample        dish (Pyrex glass Petri dish 100 mm×15 mm−top) is not statically        charged. Place the glass sample dish over the crucible and        invert crucible so that the ash is in the sample dish but        retains its shape;    -   (4) With a brush, probe, or tweezers, gently part the outer        fibers away from the center of the ash to separate those fibers        that may have been cut from those which have not. The remaining        sample should be ˜¾″ by ¾″;    -   (5) Place the crucible over the separated center portion of        uncut fibers and invert the glass sample dish over an open        plastic bag, which will catch the unwanted saw-cut fibers. Tap        the dish to dislodge the fibers that may adhere due to static        electricity;    -   (6) Examine the ash that is left. If large clumps exist, gently        separate the clump using probe tips or narrow tweezer tip;    -   (7) Assemble a vacuum filtration apparatus using a vacuum flask        ˜1500 ml, a Beuchner funnel with fixed perforated plate (diam.        60 mm) and coarse/fast flow filter paper suitable for vacuum        filtration;    -   (8) Place approximately 500 ml of water into a beaker (usually a        1000 ml beaker is used) and add ˜40 ml of glycerin. Stir to mix;    -   (9) Add glass fibers to the beaker and place beaker in        ultrasonic bath;    -   (10) Place distilled water into the ultrasonic bath so that the        beaker sits lightly in the bath;    -   (11) Turn on ultrasonic bath and wait 30 seconds, most of the        fibers will separate;    -   (12) While waiting, wet the filter paper with water and ‘seal’        it against the funnel by creating a slight vacuum. Make sure all        holes are covered by the filer paper;    -   (13) Place pipet, with an 11 mm diameter opening in the beaker        and plunge the solution in and out of the pipet ˜15 to 20 times        until all the fibers are suspended and randomized—there should        be no clumps. (A fiber-optic light, or other bright light        source, may be used to shine into the beaker and illuminate the        fibers to confirm suspension and randomization). Note—ash from        black parts should be rinsed and filtered first using a similar        procedure so that they may be observed randomizing in the        solution;    -   (14) If any clumps are seen, they should be removed from the        solution and placed in a separate beaker or dish. Add some        solution and gently separate the clump using a probe tip or        narrow tweezer tip. Return to beaker;    -   (15) Expel any solution from the pipet and move the pipet so        that the opening is in the center of the volume of solution.        Draw solution in (˜20 ml):    -   (16) Increase vacuum in filtration system and bring the pipet        over the funnel. Expel the solution in a circular motion in an        effort to spread the fibers uniformly over the filter paper;    -   (17) Rinse the fibers using methanol in a squeeze bottle. Start        from the walls of the Beuchner funnel and in a circular motion        work to the center of the filter paper. The rinse will dissolve        the glycol relatively quickly—a relatively small amount is        needed;    -   (18) The vacuum should be high enough so that the filter paper        is observed drying soon after the methanol rinse. The filter        paper should be thoroughly dry in 1 minute or less;    -   (19) Turn off the vacuum and take the funnel off of the flask        keeping it upright;    -   (20) Place a clean and static treated Petri dish over the funnel        and quickly invert so that the filter paper falls into the Petri        dish. Remove the funnel keeping the filter paper from sliding        around the dish (this action will cause the fibers to clump);    -   (21) Place the Petri dish on a flat surface and the push down on        one edge of the filter paper to secure it from moving. Grab the        opposite side with tweezers or your finger tips and lift the        filter up slightly, keeping it parallel to the bottom surface of        the Petri dish. Pull the paper taut, and then bring the opposite        sides of the paper together so that the filter paper folds        similar to a book, and snap taut to dislodge fibers from the        filter paper. Move grasp on the filter paper 45 degrees and        repeat;    -   (22) If fibers adhere to the filter paper, a soft brush (camel        hair) is used to brush them into the Petri dish;    -   (23) The sample dish should contain randomly aligned fibers and        virtually no clumps. If there are clumps: solution may have been        expelled from the pipet too fast, too much solution may have        been expelled from the pipet, the pipet was maneuvered too        slowly over the filter paper as solution was expelled, the dry        filter paper slid in the Petri dish prior to it being lifted and        ‘snapped’, or the fiber solution is not dilute enough (add        water);    -   (24) If there are too many fibers in the dish, select a second        dish and invert the first one over the second one to reduce the        number of fibers (analyze both dishes);    -   (25) Repeat until at least 3000 fibers are imaged. Add water and        glycerin to the beaker if necessary to make up for the solution        lost.

A preferred apparatus employed includes a Prior H101 motorized stage:4″×3″ travel, repeatedly ±1 μm, with controller, joystick and holder, aswell as a QIcam monochromatic digital firewire camera: 1392×1040 pixels,4.65 μm×4.65 μm pixel size, ½″ optical format Electronic Shutter,12-bit, External trigger, Zoom 70XL module with detents/iris. There isfurther provided an MND44020 Nikon focus Mount and MSS modular supportstand, a 150W halogen transmitted light source with backlight, ImageProPlus ver 6.0 software, Scope Pro plug-in module, Imaging computer:Windows XP Pro, Pentium 4 3.6 GHz processor provided with MS Office 2003Basic and Pyrex glass Petri dish 100 mm×15 mm (top only).

Preferred software to use in connection with the apparatus is FASEPVersion 1.51 Plug-In for ImagePro Plus, May, 2006, available from IDMSystems, Darmstadt, Germany. This system may be used to analyze clusterswith overlaying fibers as well as curved fibers using Hough Transformanalysis. For example, the Hough Transform may be used to compute theedge orientation histogram. The Hough Transform is a well-known methodfor finding lines. A detailed description of the Hough Transform can befound in “Digital Picture Processing”, by Azriel Rosenfeld and AvinashC. Kak, (Academic Press, Inc. 1982) Vol. 2, pp. 121-126. The HoughTransform converts an edge map image into a 2-D histogram with onedimension being the line orientation and the other being the lineintercept. Hough Transform entry HT (x,y) represents the length of aline that has an orientation of x and an intercept of y. The edgeorientation histogram H(x) can be obtained by manipulating the HT (x,y)histogram as follows:

H(x)=(ΣHT(x,y)²)^(1/2)

where the summation is over all y values.

The edge orientation (EO) algorithm is performed on the edge orientationhistogram H(x) as follows:

EO=−ΣH(x)log H(x)

The software is sometimes manually guided, and parameters adjusted sothat clusters of fibers and optionally curved fibers are properlymeasured. We refer to this procedure as an “automated” resolving processand thus this feature of the system is referred to as an automatedcluster resolving capability. See, also, U.S. Pat. No. 6,985,628 to Fan,the disclosure of which is incorporated herein by reference in itsentirety.

In general, the analysis is performed by setting a fiber diameter rangeand using Hough Transform analyses and rejecting results which areinconsistent with the physical image, discussed further below.

The system calibration is set using an NIST 25 mm stage micrometer sothat live tiling tolerance is as tight as possible. This is done bypositioning the micrometer on the stage so that it lies where framesmeet in the X and Y directions. Using the User Defined tiling method andgradient blend stitching option, set X, Y, and guard frame values sothat tiling with the algorithm results in a calibration of ±20 μm orless. (This is confirmed by imaging the stage micrometer.) Name thecalibration file and set it as the System Calibration File.

The system operates by placing the Petri dish with dispersed fibers onthe motorized stage and imaging an area approximately 65 mm×50 mm. Thespecimen is then processed as follows:

-   -   (1) Switch on power to the automated system BEFORE launching        ImagePro;    -   (2) Select Acquire and StagePro;    -   (3) Initialize stage by selecting the second radio button down        (Use physical limits of stage . . . ) and press continue on the        following dialogue box. Wait for stage to stop;    -   (4) Click on the Lens/Mag page, select your calibration file        from the drop down window. Click on Calibrate XY and then set        from file. Choose the file name of the system calibration and        click OK;    -   (5) Go to the Stage tab and set the imaging origin to the lower        right corner—use preview to see where the camera is imaging.        Click on set origin to current position;    -   (6) Set the Scan Area to 5×5 and save settings (including guard        frame) with an appropriate name;    -   (7) Place a clean, empty sample dish on the stage and choose        correct background and flat field radio buttons on the Acquire        page;    -   (8) Acquire a background image (a single image). Select the        image as the background image. Do not close the background        image;    -   (9) Switch to the sample dish and acquire the 5×5 sequence.        Select Processing from the menu bar and tile images from the        drop down;    -   (10) Choose the sequence to input and select set from frames.        Apply the user defined stitching method (previously        established). Save mosaic;    -   (11) Launch Fasep with only one image open (Fasep performs        functions on open images, only one image should be open at a        time);    -   (12) Set measurement parameters. Length minimum=0.5 mm, fiber        width is sample dependent;    -   (13) Set segmentation threshold so that the long fibers are        adequately filled in without gaps (˜205);    -   (14) Perform Blob analysis;    -   (15) Set 3 bins (dust, single fiber, cluster) for classification        using fiber width. Classify sample;    -   (16) Check classifications and modify if needed;    -   (17) Separate objects into the classes;    -   (18) After cluster images have been generated, choose one at a        time for separation analysis;    -   (19) Initially begin with Min. Maxima Area set at 4 and Avg.        Fiber Diameter set at 8 (for ˜30 μm diameter fibers). Separate        fibers using Hough Transform;    -   (20) Use Maxima click as cited in the Fasep directions when        separation is unsuccessful. Increase the Avg. Fiber Diameter as        appropriate to measure long or curved fibers;    -   (21) When all clusters are analyzed, open data collector and        export data to clipboard.

This method enables appropriate pixel size calibration for short fibersand image processing so that approximately a 65×50 mm area can beanalyzed. The process enables short and long fibers to be measured withaccuracy. The fibers in the prescribed field are automatically imagedand measured so that sampling is unbiased.

In order to produce samples made according to the present disclosure, a280 ton Nissei screw injection molding machine was used. The mold usedhad a 12 cavity mold. CELSTRAN pellets obtained from the CelaneseCorporation of Dallas, Tex. were used to form the composite foam polymersamples. The CELSTRAN pellets comprised polypropylene containing glassfibers in an amount of 40 percent by weight. The pellets had a length of½ inch. During the process, a super critical nitrogen fluid was fed tothe molding machine in order to form a cellular structure. The compositepolymer was heated to a temperature of about 420° F. during the process.

The samples made according to the present disclosure were produced usingthe injection molding screw shown in FIG. 1. During production of thesamples, the screw was rotated at 50 rpm and the system had a backpressure of 1000 psi.

When producing the comparative examples made generally according to thesystem described and illustrated in U.S. Patent Application PublicationNo. 2005/0042434, the screw was rotated at 75 rpm and at a back pressureof 2500 psi.

The following results were obtained:

Samples Made Comparative According to Present Test Samples DisclosureDrop Impact Test (ft-lb) 6.57 8.13 Notched Impact Test (ft-lb/in²) 2.352.54 Flexural Strength Test (psi) 18,695 20,101 Flexural Strain Test (%)3.26 3.23 Tensile Strength Test (psi) 11,639 12,686 Tensile Strain Test(%) 2.62 2.35 HDT Test (° C.) 148.4 149.5 Average Fiber Length (mm) 1.101.44

As shown above, samples made according to the present disclosure hadimproved properties in every category in comparison to the othersamples.

In addition to the above test, the fiber length distribution of theglass fibers contained in the polymer composite samples was alsoplotted. The results are shown in FIG. 2. As shown, the longest fiberlength recorded for the comparative sample was less than 6 mm. Thepolymer sample made according to the present disclosure, however, hadfiber lengths greater than 6 mm, such as greater than 6.5 mm, such aseven greater than 7 mm. In addition, fiber lengths overall were muchgreater for the sample made according to the present disclosure incomparison to the comparative sample.

These and other modifications and variations to the present inventionmay be practiced by those of ordinary skill in the art, withoutdeparting from the spirit and scope of the present invention, which ismore particularly set forth in the appended claims. In addition, itshould be understood that aspects of the various embodiments may beinterchanged both in whole or in part. Furthermore, those of ordinaryskill in the art will appreciate that the foregoing description is byway of example only, and is not intended to limit the invention sofurther described in such appended claims.

1. A polymer composite article comprising: a polymer matrix comprising athermoplastic polymer, the polymer matrix having a cellular structure;and fibers dispersed in the polymer matrix, the fibers being present inthe polymer matrix in an amount of at least about 10 percent by weight,wherein at least 10 percent of the fibers by volume have a length ofgreater than about 3 mm and wherein at least about 10 percent of thefibers by volume have a length of from about 3 mm to about 4 mm. 2.(canceled)
 3. A polymer composite article as defined in claim 1, whereinat least some of the fibers have a length greater than 6 mm.
 4. Apolymer composite article as defined in claim 1, wherein at least 1percent of the fibers by volume have a length of greater than about 7mm.
 5. A polymer composite article as defined in claim 1, wherein thefibers comprise glass fibers.
 6. A polymer composite article as definedin claim 1, wherein the thermoplastic polymer comprises a polyolefin, apolyamide, or mixtures thereof.
 7. A polymer composite article asdefined in claim 1, wherein the article has a void volume of at leastabout 5 percent.
 8. A polymer composite article as defined in claim 1,wherein the article has a void volume of at least about 10 percent.
 9. Apolymer composite article as defined in claim 1, wherein the polymermatrix contains a plurality of cells, the cells having an average cellsize of less than about 100 microns.
 10. A polymer composite article asdefined in claim 1, wherein the article has been injection-molded.
 11. Apolymer composite article as defined in claim 1, wherein the polymermatrix has a cell density of at least about 10⁶ cells per cubiccentimeter.
 12. A polymer composite article as defined in claim 1,wherein the article has a drop impact of greater than about 7 ft-lb andhas a notched impact of greater than about 2.5 ft-lb/in².
 13. A polymercomposite article as defined in claim 1, wherein the fibers dispersed inthe polymer matrix have an average fiber length of greater than about1.25 mm.
 14. A polymer composite article as defined in claim 1, whereinthe fibers comprise talc fibers, wollastonite fibers, carbon fibers,metal fibers, aromatic polyamide fibers, or mixtures thereof.
 15. Apolymer composite article as defined in claim 1, wherein the fibers arepresent in the polymer matrix in an amount from about 20% to about 60%by weight.
 16. A polymer composite article comprising: a polymer matrixcomprising a thermoplastic polymer, the polymer matrix having a cellularstructure; and fibers dispersed in the polymer matrix, the fibers beingpresent in the polymer matrix in an amount of at least about 10% byweight, and wherein at least 1 % of the fibers by volume have a lengthof greater than about 6.5 mm, and wherein at least about 1 percent ofthe fibers by volume have a length of from about 3 mm to about 7 mm. 17.A polymer composite article as defined in claim 15, wherein at leastabout 10% of the fibers by volume have a length greater than about 3 mmand wherein the fibers have an average length of greater than about 1.25mm.
 18. A polymer composite article as defined in claim 16, wherein thearticle has a void volume of at least about 5%.
 19. A polymer compositearticle as defined in claim 16, wherein the thermoplastic polymercomprises a polyolefin, a polyamide, or mixtures thereof and wherein thefibers comprise glass fibers.
 20. A polymer composite article as definedin claim 16, wherein the article has a drop impact of greater than about7 ft-lb and has a notched impact of greater than about 2.5 ft-lb/in².21. A polymer composite article as defined in claim 16, wherein thearticle has been injection-molded.